By unlockable characters, they didn't mean that you'd be able to play as them. In the GameSpy interview, it was stated that they wanted you to be "the grunt out on the front lines". I believe the "unlockable characters" will fight along side you, and go and do all the heroic stuff for continuity's sake

I also get the feeling that most of the vehicle's we've come to love will be in there, if they do the Battle of Geonosis right, it should be pretty spectacular for vehicular action